But that would supposed need to have some explicit text stating the expiration of that contract. An existing contract can't just end when provider feels like it, I suppose?
Every contract has that, either party can exit the contract under normal conditions. You can cancel your Netflix subscription with a short notice period. They can do the same. They use the notice period as grace period for you to accept the new conditions. You accept a new contract with new conditions.
I negotiated my mobile phone or internet contracts again and again, to get better deals. I threaten to leave, they throw a bone. Because they know I have options. Providers who know you don’t will squeeze you however they please.
I would guess it can end the moment either party wants, unless a length was established. At the end of the month or year you’ve paid for, perhaps with a minimum notice, would make sense. Otherwise the provider can refuse to let you stop paying, citing the contract.